The present invention relates generally to the field of forming a bell or shaped socket in the end of a length of plastic pipe, and more particularly, is directed to a novel method and apparatus capable of producing a sharply defined bell on a previously softened end of a length of plastic pipe regardless of wall thickness.
It is the present practice to utilize known types of plastic pipe forming apparatus to provide a bell or shaped socket in one end of each length of plastic or other pipe to allow joints in adjacent lengths of pipe to be readily made in the field by utilizing known techniques and existing tools. The presently available pipe belling machines for use with plastic pipes usually incorporate a means to heat one end of the plastic pipe prior to belling to sufficiently soften the plastic to permit shaping the bell. The existing belling apparatus utilize a mandrel means to shape the interior of the bell at the heated pipe end and external clamps to shape the exterior periphery of the belled end during the belling process. It is also common practice to reciprocate the mandrel means into and out of the end of the pipe during the belling process. The prior art devices usually also incorporate suitable cooling means to set the shaped end after the withdrawal of the mandrel means in a rapid manner to provide efficient utilization of the machine and to reduce time requirements and production costs.
It is now increasingly popular and usually necessary to provide an interior groove in the shaped bell as the pipe bell is formed to accommodate a sealing gasket. The sealing gasket is secured within the groove and is employed on the job site to quickly and automatically seal the pipe joint when adjacent lengths of pipe are joined together. To form such an internal groove in the pipe bell, it is now the common practice to provide a plurality of expanding and contracting cooperating segments within a mandrel construction to thereby permit the mandrel to automatically and simultaneously form the interior configuration of the pipe bell and also to form the interior peripheral groove.
It will be appreciated that wall thicknesses of plastic pipes vary widely from pipe to pipe due to the design requirements of such parameters as pipe diameter, type of intended service of the pipe, the type of plastic employed in fabricating the pipe, pressure requirements of the finished pipeline and other such design considerations. Due to the variances in wall thickness of the different plastic pipes which are now presented to a belling fabricator for pipe belling purposes, each mandrel apparatus for each diameter of pipe has to be equipped with various sized external clamps, which clamps are specifically designed and configured for a particular pipe wall thickness. Accordingly, it is possible for one mandrel to require several sets of clamps in order to make a pipe belling machine relatively universally adaptable for the pipes normally treated by a single plant. In other instances, additional tooling costs were involved in carefully machining the clamps to assure forming bells with minimum acceptable wall thickness throughout the entire bell, including the critical area in and about the internal groove. Because of this, tooling and equipment costs have become quite expensive when utilizing existing machines to form pipe bells by employing the combination of a mandrel with external clamps.
Additionally, the external clamps have proved to be deficient to a degree in that the use of such external clamps has usually resulted in marring the exterior periphery of the belled end of the pipe due to imperfections in the surfaces of the clamps, and imperfections in the mating surfaces of the cooperating upper and lower clamp halves possibly caused by misalignment of the parts during the bell forming operations.